Often considered a result of specific line crosses and inbreeding, naval rupture has been noted in several breeds, including the Friesen Horse. Naval rupture is often called umbilical hernia, and can be serious is not treated. It is estimated that about one to two percent of all foals born have some type of umbilical hernia or naval rupture, not all which are obvious or serious.

The naval rupture occurs when the abdominal walls around the umbilical cord do not form correctly, leaving a hole or weak area around the naval cord. When the foal is born the pressure through the birth canal as well as the act of struggling to stand can force loops of the intestine or fatty tissue down through the abdominal wall to form a bulge at the naval. This can happen in both male and female foals, although male foals also run the risk of having scrotal hernias as well.

Umbilical hernias are considered to be reducible if the intestines or the fatty tissue can be moved back up into the abdominal area. In new born foals this is most easily accomplished by rolling the foal onto his or her back and gently applying the flat of the hand to the hernia, gently pushing the mass back up into the abdomen. Often in many foals the pressure from the hand won't be required, the simple act of rolling the foal over will cause the intestine to return to the correct position. It may be necessary to repeat this process as required, but usually by about six months of age the abdominal wall is strong enough to hold the intestines in place and the rupture will heal naturally, preventing any further problems. Abdominal wraps may also be used to keep the hernia retracted until the abdominal wall repairs itself. Irritants such as iodine can be injected into the hernia area to increase the tissue repair in this location.

In cases where the hernia or rupture is large or the mass does not go back into the abdominal area it is important to seek veterinary assistance immediately. The intestine can become twisted, cutting of blood supply and interrupting the passage of gas and waste material. This will result in the foal showing signs of colic and pain including kicking at the stomach, rolling over and increased in the swelling of the hernia. In cases where the blood flow is partially or completely blocked the hernia is called a strangulated hernia and is life threatening for the foal.

Surgical procedures to correct a strangulated hernia are often required, especially if the foal has developed the colic like symptoms. Without surgery the condition can become fatal, especially if the blood supply is cut off to the rest of the digestive tract. Surgery should only be done when all other methods have failed or to work or when strangulation of the hernia is confirmed.